This object specifies a bit rate threshold. When the
following condition is met, the agent generates an
adslAtucRateChangeTrap:
( I + F ) < cAdslAtucDmtThreshRateFallback
where I = adslAtucChanCurrTxRate for ATU-C's interleave channel
F = adslAtucChanCurrTxRate for ATU-C's fast channel
Setting this value to '0' prevents the agent from generating an
adslAtucRateChangeTrap due to rate fallback.